Post-synthetic modified luminescent metal–organic framework for the detection of berberine hydrochloride in a traditional Chinese herb

In this work, a novel fluorescence sensor UiO-66-PSM based on post-synthetic modified metal–organic frameworks was prepared for the detection of berberine hydrochloride (BBH) in the traditional Chinese herb Coptis. UiO-66-PSM was synthesized by a simple Schiff base reaction with UiO-66-NH2 and phthalaldehyde (PAD). The luminescence quenching can be attributed to the photo-induced electron transfer process from the ligand of UiO-66-PSM to BBH. The UiO-66-PSM sensor exhibited fast response time, low detection limit, and high selectivity to BBH. Moreover, the UiO-66-PSM sensor was successfully applied to the quantitative detection of BBH in the traditional Chinese herb Coptis, and the detection results obtained from the as-fabricated fluorescence sensing assay were consistent with those of high-performance liquid chromatography (HPLC), indicating that this work has potential applicability for the detection of BBH in traditional Chinese herbs.


Introduction
Berberine hydrochloride (BBH) is an isoquinoline alkaloid extracted from traditional Chinese herbs Coptis, Cortex phellodendri, etc.A large number of studies have shown that BBH has a variety of pharmacological effects, 1 including anti-pathogenic microorganisms, anti-inammatory, antitumour, cardioprotective, hypoglycaemic, lipid metabolism regulation, and immunosuppression.Various methods have been reported for the detection of BBH, including chromatography, 2 colorimetry, 3 electrochemistry, 4 and uorescence. 5Compared with other methods, uorescence is a favourable method for the analysis of BBH due to its simple operation, high sensitivity, and better selectivity.To date, several uorescent sensors have been developed for the detection of BBH, including carbon dots, 6 quantum dots, 7 metal nanoclusters 8,9 and metal-organic frameworks (MOFs). 10MOFs have been widely used in sample pretreatment, [11][12][13] catalysis, 14 drug delivery, 15 uorescence bioimaging, 16 and electrochemical and uorescence sensing [17][18][19] based on their controlled pore size, large specic surface area and tunable molecular structure.1][22][23] A literature survey indicates that MOFs-based uorescence sensors used for the detection of BBH are sparse except for the following report.Xiong et al. 10 prepared a microscale highly uorescent Eu metal-organic framework, which was used as a dual-mode visual sensor for the sensitive detection of BBH and tetracycline.Therefore, it is very signicant to develop MOFs-based materials for highly sensitive and selective detection of BBH.
5][26] LMOFs sensors have made some progress in the detection of anions, 27 cations, 28 volatiles and gas molecules. 29owever, it is still a great challenge to prepare an ideal MOFs structure with the desired performance and functionality.Direct doping and post-synthetic modication (PSM) have become effective strategies to overcome this problem; 30,31 in particular, PSM has been widely applied in the sensing eld because it only occurs on the frame or within the aperture, and does not damage the crystal structure.For instance, in our group's previous work, Yu et al. 32 synthesized a highly luminescent MOF (TMU-PC) by covalent PSM of TMU-17-NH 2 with 2pyridinecarboxaldehyde for rapid detection of inorganic pyrophosphate in human urine and synovial uid samples.
As a typical representative of the LMOFs family, UiO-66 series functional materials have been applied in uorescence sensing based on the advantages of large specic surface area, unsaturated metal sites, functionalized framework structure,

RSC Advances
PAPER good stability, etc. 33,34 The most common is UiO-66-NH 2 with a weak blue uorescence, which is prepared by organic ligand containing chromophore amino group. 35The amino group in UiO-66-NH 2 is not only an active but also a basic group, which can undergo condensation reaction with the active carbonyl group via covalent PSM, i.e.Schiff base reaction. 36ere, in this work, phthalaldehyde (PAD) was selected as the modier to react with UiO-66-NH 2 to fabricate a novel uorescent sensor, named UiO-66-PSM (Scheme 1).To detect BBH sensitively and selectively, the prepared UiO-66-PSM sensor was designed considering the following two reasons.On the one hand, UiO-66-NH 2 has unsaturated metal sites (Zr) that can interact with the electrons of the oxygen atoms on BBH.On the other hand, the carboxy of UiO-66-PSM makes it tend to form electrostatic interaction with BBH.The developed UiO-66-PSM material has the advantages of fast response, strong speci-city, and high sensitivity as a BBH uorescence sensor.The effectiveness of the method was veried by the detection of BBH in the traditional Chinese herb Coptis.To the best of our knowledge, this is believed to be the rst example of a MOF-PSM sensor being used for the analysis of traditional Chinese herbal ingredients.

Materials and reagents
Zirconium chloride (ZrCl 4 ), 2-amino-1,4-benzenedicarboxylic acid (NH 2 -H 2 BDC), and PAD were purchased from McLean Reagent (Shanghai, China).Berberine hydrochloride was obtained from Shanghai Yuanye Bio-Technology Co., Ltd.All chemicals were analytical grade or better, and the water used in all experiments was ultra-pure water.

Instrumentation
UV-Vis absorption spectrum was determined by a Shimadzu UV-2550 spectrophotometer.The uorescence emission spectra were obtained using a Hitachi F-2700 spectrouorophotometer.Fourier transform infrared (FT-IR) spectra were measured by Shimadzu Tracer-100 FT-IR Spectrometer.Morphological evaluation was carried out by scanning electron microscopy (ZEISS GeminiSEM 300).Energy dispersive X-ray spectroscopy (EDX) was carried out by Rigaku Smartlab 3 KW.Thermogravimetric analysis (TGA) was carried out by TA Q500.Fluorescence decay curves were obtained by Edinburgh FLS1000.Elemental compositions were detected by Thermo Scientic K-Alpha.BET surface area and the pore volume were obtained by Mike ASAP2460.

Synthesis of UiO-66-PSM
UiO-66-NH 2 was prepared according to the method in the ref.34.The prepared UiO-66-NH 2 was dissolved in ethanol (65 ml), PAD (0.6075 g) was added, aer which acetic acid was dropped into the UiO-66-NH 2 solution and sonicated for 5 min.Then the suspension was reuxed at 80 °C for 24 h under a nitrogen atmosphere.The obtained khaki solid was washed with ethanol and dried in a vacuum at 60 °C overnight.

Fluorescence detection of BBH
UiO-66-PSM (2 mg) was dispersed in phosphate buffer solution (PBS, pH = 7.4) and sonicated for 5 min to obtain a homogeneous suspension, aer which various concentrations of BBH were added for uorescence measurement.The uorescence excitation wavelength was 329 nm and the emission spectra were scanned in the range 349-670 nm.

Real samples preparation
A sample of Coptis powder (0.2 g) was passed through a 24 mesh sieve and placed in a conical ask, a mixture of methanol and hydrochloric acid (V/V = 100 : 1, 50 ml) was added, and the total weight was weighed.Ultrasonic extraction was performed on this mixture for 30 min at room temperature.The mixed solution was weighed again and methanol was added to make up for the lost weight.Then the obtained solution was thoroughly shaken and ltered.The subsequent ltrates (2 ml) were collected and xed in a 10 ml volumetric ask with methanol.Finally, the yellow extract was shaken well and ltered, the subsequent ltrates were collected once again.
The elemental composition of UiO-66-NH 2 and UiO-66-PSM were studied by X-ray photoelectron spectroscopy (XPS).The signals at 531.08, 399.08, 284.08, and 182.08 eV in the spectra corresponded to O 1s, N 1s, C 1s, and Zr 3d, respectively. 38ompared with UiO-66-NH 2 , the additional peak at 399.7 eV in the spectrum of UiO-66-PSM was assigned to amide N (Fig. 1B), suggesting that PAD was successfully assembled on the skeleton of UiO-66-NH 2 .FT-IR spectra of UiO-66-NH 2 and UiO-66-PSM were presented in Fig. 1C.As can be seen from Fig. 1C(a), the bands at 3317 cm −1 and 3371 cm −1 belonged to the symmetrical and asymmetrical stretching vibrations of N-H, respectively.However, a broadband replaced the peak of amine functional groups in the UiO-66-PSM spectrum, and a peak at 3405 cm −1 was attributed to hydroxyl groups.Furthermore, the increased band at 1746 cm −1 belonged to the C]O of the carboxyl group (Fig. 1C(b)).
Emmett-Teller (BET) surface areas and porosities of UiO-66-NH 2 and UiO-66-PSM were obtained through the measurement of nitrogen (N 2 ) adsorption-desorption isotherms (Fig. 1D).The BET surface area of UiO-66-PSM (774 m 2 g −1 ) was smaller than that of UiO-66-NH 2 (974 m 2 g −1 ), which may be due to the introduction of PAD. 39According to the IUPAC classication, 40 the curve was consistent with type I isotherms, N 2 adsorption on UiO-66-NH 2 and UiO-66-PSM was dramatically upward at the low pressure region, indicating that they were microporous materials.
Thermogravimetric analysis (TGA) was carried out to study the thermal stability of UiO-66-NH 2 and UiO-66-PSM in the range of 25-1000 °C.As shown in Fig. 1E, there was an initial weight loss before 340 °C, possibly due to the decomposition of residual solvents in the framework. 41The weight loss from 340 to 630 °C was attribute to the thermal dissociation of the UiO-66-NH 2 .As shown in Fig. 1F, the weight loss from 340 to 620 °C may be due to the decomposition of PAD modied on UiO-66-NH 2 .The weight loss from 630 to 960 °C might come from the framework collapse of UiO-66-PSM.The above results indicated that UiO-66-PSM had high thermal stability.

Fluorescence properties of UiO-66-PSM
To study the uorescence properties of the sensor, the excitation and emission spectra of UiO-66-PSM were detected (Fig. 2A).UiO-66-NH 2 demonstrated a uorescence emission at 419 nm under excitation at 373 nm, which was ascribed to ligand-to-metal charge transfer (LMCT). 35Aer modication, UiO-66-PSM demonstrated a uorescence emission at 425 nm under excitation at 329 nm, and exhibited a bright blue uorescence than UiO-66-NH 2 under 365 nm UV lamp (Fig. 2B), which may be related to the increase of the conjugation degree and LMCT efficiency of UiO-66-PSM caused by the introduction of PAD (Fig. S2 †).
The inuence of time and pH on the uorescence intensity of the UiO-66-PSM sensor was investigated.As shown in Fig. S3, † the uorescence intensity of UiO-66-PSM rstly increased and then decreased with the increase of pH, and reached the maximum when pH value was 7.4.The internal charge transfer process in the UiO-66-PSM framework was responsible for this pH-dependent uorescence property.The sensor showed high uorescence stability within 2 weeks, the change of uorescence intensity was negligible (Fig. 2C).Moreover, the inset in Fig. 2C showed that the uorescence of the sensor remained stable aer 60 min of continuous exposure to a 365 nm UV lamp.

Sensing of BBH
Based on the above uorescence characteristics of UiO-66-PSM, we explored the uorescence sensing of BBH.The uorescence response rate of UiO-66-PSM to BBH was studied.As shown in Fig. 3A, the uorescence intensity of the sensor decreased rapidly aer the addition of BBH and stabilized aer 30 s.Compared with traditional chromatography, the sensor developed in this work responds faster to BBH, and has great potential in the detection of traditional Chinese herb.
As shown in Fig. 3B, the blue uorescence of UiO-66-PSM was obviously quenched aer the addition of BBH.It can be seen from Fig. 3C that the uorescence intensity of UiO-66-PSM gradually decreased with the increase of BBH concentration.When 0.1 mM BBH was added, more than 90% of the uorescence was quenched.The uorescence intensity ratio (I 0 /I) of the sensor was linear within the BBH concentration from 3.3 × 10 −3 to 0.66 mM.The linear regression equation is y = 44.01x+  0.6278 (R 2 = 0.9992) (Fig. 3D).The limit of detection (LOD) can be calculated using the Stern-Volmer (SV) equation: where K sv (M −1 ) is the Stern-Volmer quenching constant, [Q] is the concentration of BBH, I 0 and I are the luminescence intensities in the absence and presence of BBH, respectively.According to the above equation, the K sv was calculated to be 4.35 × 10 4 M −1 , and the LOD was determined to be 0.096 mM based on the 3s rule. 42Besides, a comparison between the uorescence sensor developed in this work and other methods reported for the detection of BBH is listed in Table S1, † indicating that this method has a greater advantage in terms of response speed and linear range.
To better evaluate the selectivity of UiO-66-PSM to BBH, we determined the uorescence intensity of UiO-66-PSM aer adding various potential interferences, including glycine, L- lysine, L-cystine, L-histidine, glucose, sucrose, K + , Na + , Ca 2+ , Mg 2+ . 6The uorescence quenching was performed by 6.6 × 10 −4 M BBH and its mixtures with 6.0 × 10 −3 M other various potential interferences.The above interferences did not cause signicant changes in uorescence intensity (Fig. 3E).Such results indicate that the UiO-66-PSM sensor exhibits merits of specicity and anti-interference ability to BBH, so that the sensor could achieve quantitative detection of BBH.

Analysis of real samples
To investigate the practical application of the developed UiO-66-PSM sensor in the real sample, we determined BBH in the traditional Chinese herb Coptis.The determination results of BBH were conrmed by HPLC method (Fig. S4 †).As shown in Table 1, the results obtained by this method were close to those obtained by HPLC.Coptis spiked with different concentrations of BBH were investigated, the recoveries ranged from 99% to 103.1%, and the RSD was less than 2.0 (n = 3).The above results conrm that the UiO-66-PSM sensor is effective and practical for the detection of BBH in traditional Chinese herb.

Investigation of uorescence sensing mechanism
In general, there are several main reasons for uorescence quenching in MOF-based sensors: collapse of the MOF framework, 43 uorescence resonance energy transfer (FRET), 44 and photo-induced electron transfer (PET). 45As can be seen from Fig. 4A, the diffraction peaks in the PXRD spectrum of UiO-66-   PSM were approximately unchanged aer the introduction of BBH, implying that the structure of UiO-66-PSM was not destroyed during the quenching process.FT-IR spectra of the sensor before and aer the addition of BBH were shown in Fig. 4B.Aer adding BBH, the peak of C]O had a blue shi from 1657 cm −1 (curve a) to 1620 cm −1 (curve b), implying the interaction between the carboxylic acid group of UiO-66-PSM and the N of BBH.Therefore, the interactions could be speculated as follows: the coordination between the oxygen atoms of BBH and unsaturated metal sites (Zr); the acid-base interaction between the carboxylic acid group on UiO-66-PSM and the N of BBH.Due to BBH was immobilised and the distance between the ligand and BBH was close, the energy absorbed by the ligand could be transferred to BBH, which reduced the ligand-to-metal energy transfer efficiency and led to the quenching effect on the uorescence intensity of UiO-66-PSM. 46Thus, the uorescence quenching mechanism of UiO-66-PSM should be PET.We further investigated the uorescence quenching mechanism of UiO-66-PSM towards BBH.The uorescence emission spectrum of UiO-66-PSM and the UV-vis absorption spectrum of BBH were measured.As shown in Fig. 4D, the UV-vis absorption of BBH did not overlap with the emission of UiO-66-PSM, thus the FRET mechanism was excluded.
The mechanism of uorescence quenching can be divided into dynamic quenching and static quenching. 47Time-resolved uorescence decay experiments were performed.As shown in Fig. 4C, the uorescence lifetimes of UiO-66-PSM in the presence and absence of BBH were 8.69 ns and 8.58 ns, respectively.The uorescence lifetime of the sensor was essentially unaffected by the introduction of BBH, demonstrating that the detection mechanism was static uorescence quenching.

Conclusion
In summary, we designed and synthesized a uorescent sensor (UiO-66-PSM) in a relatively short time, and applied it to the quantitative detection of BBH.The developed sensor exhibited high sensitivity (0.096 mM), fast response time (30 s) and excellent selectivity.In addition, the sensor was successfully applied to detect BBH in the traditional Chinese herb Coptis.It is expected that the MOF-based sensor material will have great application prospects in the detection of other ingredients in traditional Chinese herb.

Table 1
Determination results of BBH in Coptis samples using the proposed method and confirmed by HPLC method and precision of the proposed method (n = 3) a a HPLC: the amount of BBH determined by HPLC.